July 8, 2020 | Daywey Chen
2020-07-08

Communication has enabled society to advance. To be able to commutate effectively and efficiently has separated humans from all other species on the planet. People commutate in text, in sound, in code, in pictures…etc. People commute in many different languages, English, Chinese, Spanish, Malaysian…etc., you name it. As a printing professional, how do we communicate in color? What is the common color language that enables us to be on the same page?

To begin with, let's define what is color? Color is defined as the perception of light that has been modified by an object. There are mainly three elements that come into play when determining color, which are light source, object, and the perceptive human eye or instrument. Light is broken down into different categories by its wavelength and frequency. For instance, cosmic and gamma rays x- rays, ultraviolet radiation, infrared radiation, heat, radio waves. The human eye can perceive light within a certain wavelength range, we call this range the visible range or visible spectrum. The visible range light source will consist of different wavelengths of different intensity. When the light is shone onto an object, the object will absorb a certain wavelength and reflect others. The reflected wavelength will be received by the human eye or instrument and perceived as a certain color. For instance, a leaf appears green because it absorbs the red and blue wavelengths and reflects the green wavelength.

 

The human color vision is broken down into the red, green, and blue regions. The eye has three sensors or receptors that detect these three primary colors. All colors perceived is a mixture of these primary colors. Different light sources consist of different intensity of the red, green, and blue wavelength. Therefore, the perceived color will be different under different light source. For instance, the sunlight on a perfect daylight has lower blue intensity therefore the object will appear less blue. The incandescent light has a higher red intensity therefore objects will appear redder under incandescent light. The fluorescent light has a greater blue intensity, therefore, making the object look bluer. With the above being said, the first step to communicate in color is to judge the color of the object from a consistent light source. Sunlight is the most natural way to view objects. However, it is not an ideal light source to judge the color as it varies too much. Artificial light sources are recommended to do the job as it is much more controlled.

 

Now, even with the same light source, humans still perceive color slightly differently. The eye has photoreceptor cells called cones. The cones are divided into three types which are the S-Cones that are sensitive to short-wavelength light (blue), M-Cones that are sensitive to medium wavelength light (green), and the L-Cones that are sensitive to long-wavelength light (red). Different individuals’ cones have different sensitivity. For some less fortunate individuals with defective cones are described as color blind. As different individuals will perceive color slightly differently, we need a color system to quantify color in a mathematical way.

 

A common color system used in the industry is the CIE perceptual color space. The CIE system is based on the L*a*b* model (Figure 1). L* stands for lightness and is the vertical dimension in the color space. For a* and b* is the x and y coordinates of the chromatic plane. The chromatic plane is a cross-section of perceptual space viewed from the top as a two-dimensional plane. Each visible color has a L*a*b* value assigned to it. Another model that can describe a color is the L*C*h model (Figure 2). The L*C*h model uses polar coordinates to locate color in its color space. L* stands for lightness and is still the vertical dimension. Instead of using an x and y coordinate, the L*C*h model uses C* as the length, and h as the degree angle to identify the color location on the two-dimensional plane. C*, or chroma refers to the saturation of the color. A C* value of 0 is a color of neutral or gray. The more saturated or pure the color, the higher the C* value. The h value is the perception of the color, it determines whether the color is red, or green or purple or …etc.

Figure 1: 


Figure 2:


Now we have a system to quantify and identify color. During the printing operation, there will always be a difference between the target image and the actual image. How do we communicate the difference between the target and the actual? To measure the difference between two colors we use something called the delta E or
E. E. is the geometric distance between two colors. The general mathematical formula can be described as E = sqrt((L1-L2)^2 + (a1-a2)^2 + (b1-b2)^2.) However, specifying an acceptable E value is not that simple. The same E value in the color space will be perceived differently under the human eye. For instance, the same E value in neutral gray will be more apparent than in a saturated dark red. To overcome this there are different weighting calculations that are implemented to the E equation. Some common weighting method includes CMC CIE94, and CIE2000. 
For instance the CMC is E = sqrt(∆L/lSL)^2 + (∆C/cSc)^2 + (∆H/Sh)^2), click here for the full mathematical formula of the E based upon CMC.

When communicating in color, it is important that all parties use the L*a*b* model or the L*C*h model to describe the color. When setting standards, to have a meaningful and effective discussion, all parties measuring color must use the same tolerancing equation under the same light source.

Daywey Chen

Article by Daywey Chen, KYMC